Image array sensors are used extensively in fax machines, still or video cameras, sensors, scanners, telescopes and the like. The image array sensor field is dominated by two technologies, namely CCD (Charge Coupled Device) and CMOS (Complementary Metal Oxide Semiconductor). Sensitive, high pixel count sensors are expensive.
Light is one form of radiant energy which may be considered as an alternating electromagnetic radiation at very high frequency. Humans perceive different light frequencies as different colors, and there is a large amount of radiation that is not perceived by humans, generally known as UV (Ultra Violet), and IR (Infra Red), and the term light will be extended thereto. Visible light ranges generally between 760-300 nm and corresponds to the peak intensity of solar radiation transmitted through the atmosphere. Infrared radiation ranges from the extreme far end of 10 μm (33 THz; millimeter radio waves) to about 760 nm and solar radiation contains a significant amount of total energy below about 3 um. Terrestrial objects at temperatures above 300K have useable energy density at wavelengths below about 10 μm (frequencies above about 33 THz). Photons at wavelengths larger than about 10 μm have energies below about 0.125 eV and are widely considered impractical for photovoltaic detection, but may be detected and harvested utilizing electromagnetic principles.
UV radiation ranges between the visible light and higher frequency electromagnetic energy, such as X-Rays and gamma rays. While large amounts of UV energy are absorbed in the atmosphere, this radiation may still be of interest for harvesting above about 300 nm.
It is clear therefore that radiant energy as used herein covers a very broad spectrum of radiation. Clearly specific applications would be required to cover only portions of this spectrum. By way of example, for solar energy applications the spectral range of interest will likely be a spectrum containing most if not all of the solar spectrum available at the location where the solar cell is to be deployed, or the portion thereof which is economically used by the device at hand, typically 3 μm to 300 nm for example. The spectral range of interest for image array, focal plane sensors may vary from visible light only when used in cameras, to the infra-red and possibly to some part of the visible spectrum in heat sensing devices, to the whole spectrum in certain hyper-spectral applications. In some applications a specific wavelength may be desirably attenuated, such as by way of example reduction of blue light for pilot related devices. Yet, for devices directed to heat energy recovery, it is likely that only the infra-red portion of the spectral range is of interest. It is seen therefore that the application at hand determines the spectral range of interest for which an apparatus utilizing the invention is directed to.
Therefore, the spectral range of interest is defined herein as relating to any portion or portions of the total available spectrum of frequencies which is being utilized by the application and or apparatus at hand, and which is desired to be detected and/or emitted utilizing the technologies, apparatuses, and/or methods of the invention(s) described herein, or their equivalents.
For brevity and improved clarity the term ‘light’ as used herein is equivalently directed to being but one example of the frequency within the spectral range of interest, and inclusive thereof, unless specifically limited, such as ‘visible light’, infra red light, and the like.
Light may also be considered as a flow of photons, which are quantized units of energy which increases with frequency. Therefore certain terms that are common to simple electromagnetic energy need to be better specified as relating to the spectral range of interest. Thus, a dielectric material in the above mentioned energy spectrum may be defined as a material having low conductivity, and having a band-gap between a filled valence band and an empty conduction band exceeding the energy of any photon in the spectral range of interest to a specific application. A “semiconductor” refers to a photovoltaicly active material, having a bandgap comparable to or smaller than the photon energy of any photon in the spectral range of interest to a specific application. It is explicitly noted that a material that is a dielectric in one range of wavelengths may be an intrinsic semiconductor in a shorter wavelength region of the spectrum. Therefore the classification of a material as dielectric or semiconductor is determined by the structure and the frequencies at which it is intended to be used. Given the wide range of spectrum of interest in at least some applications the same material may be considered a dielectric in one location and a semiconductor in another.
In contrast, a transparent conductor is a material having a finite but meaningful conductivity due to a partially filled conduction band or partially empty valence band but having a band-gap between the valence band and conduction band exceeding the energy of any photon in the spectral range of interest. These materials act like a dielectric at some frequencies and like a semiconductor at even higher frequencies, but act like a conductor at low frequencies. Transparent dielectric materials also have low optical losses such that photons efficiently transmit through such material, at least at the spectral range of interest or a significant portion thereof for which they are employed.
While transparent conductors may be considered as wide bandgap semiconducting materials, they are used as conductors in most applications. Dielectrics, transparent conductors, and semiconductors, as used in these specifications, refer to materials that have a dielectric constant at optical frequencies; however the distinction between a semiconductor and the remaining materials is that the bandgap of a semiconductor is not substantially larger than the photon energy. As a general and non-limiting guideline, table 1 describes several characteristics of the different conductive, insulating, and semi-conductive materials.
TABLE IMaterialMetalTrans. Cond. SemiconductorDielectricBandgap→ 0>> photon≦ photon>> photonDC ConductivityhighgoodVaries→ 0Optical PropertyreflectivetransparentabsorptivetransparentDielectric constantcomplexlow losslossylow loss
The term stationary resonance condition should be construed as relating to a situation in a waveguide where the frequency of the guided wave is sufficiently close to the local cutoff frequency of the waveguide, such that the guided wave reflects repeatedly between opposing surfaces of the guide. The corresponding energy velocity along the waveguide propagation axis is significantly lower than the speed of light in the bulk material of the waveguide and approaches zero at the stationary resonance condition. Notably, complete stationary resonant condition is an ideal limiting case which is almost never achieved.
The term Continuous Resonant Trap Refractor (CRTR) should be construed as relating to a tapered waveguide having a base face and a tip. The larger face of the tapered waveguide core will be generally referred to as the aperture, and the smaller face, or point, will generally be referred to as the tip. Light travels along the depth direction extending between the aperture and the tip, however the light may travel towards the aperture, or away therefrom. For the purposes of these specifications, the depth increases from the aperture towards the tip, such that larger depth implies greater distance from the aperture. The term ‘tapered waveguide’ requires only that the waveguide core be tapered, and the overall dimensions or shape of the CRTR may be of any convenient shape.
A distance from the aperture along the depth dimension at which the width of the waveguide in at least one dimension would be the critical width which will block light of a given frequency from advancing further down towards the tip, is referred to in these specification as ‘emission depth’ for this frequency. The width of the CRTR core which causes the energy to be emitted through the cladding for a wave of a given frequency, is termed ‘emission width’ for that wave. Such emission is termed ‘cladding penetration state’. When polychromatic light is admitted through the CRTR aperture, lower frequency waves will reach their emission depth before higher frequency waves. As the wave energy departs the CRTR at its emission depth, lower frequency light would penetrate the cladding and exit at a shallower depth than higher frequency light. Thus, the CRTR will provide spatially separated spectrum along its cladding. In addition the CRTR refracts the spatially separated light away from the axis of the CRTR extending from the aperture toward the tip.
Cladding penetration may be caused by the wave approaching stationary resonant condition, or when the wave reaches a critical angle which depends on the core/cladding interface. In certain embodiments cladding may be removed at a predetermined depth to achieve desired propagation characteristics.
Cladding may comprise a dielectric material with lower refractive index than that of the core, a thin conductive layer with thickness comparable to the skin depth of the conductor, or a conductive layer with perforations. Such cladding and core systems offer total internal reflection for sufficiently thick claddings. As the wave is slowed the equivalent incident angle of the wave against the core/cladding boundary increases and penetration into the cladding increases up to the critical angle. For intermediate cladding thicknesses Frustrated Total Internal Reflection (FTIR) occurs just before the critical angle. For sufficiently thick claddings the wave is bound until the critical angle. Metallic claddings with small perforations or with thicknesses at or near the skin depth also have angle dependent reflection coefficients, resulting in a situation analogous to FTIR, and are thus also considered suitable.
CRTR's were first described in U.S. provisional patent application No. 61/701,687 and later in more detail in U.S. provisional patent applications No. 61/718,181, and 61/723,832 to the present inventors. The 61/701,687, 61/718,181, and 61/723,832 patent applications are incorporated herein by reference in their entirety.
Individual devices for converting radiant energy to electric signals shall be generally referred to hereinunder as detectors. Commonly, detectors are closely coupled to a single pixel and correspond to a single color. An array sensor relates to a linear or an area array of pixels, each utilizing at least one refractor, preferably CRTR, and a plurality of coupled detectors, each detector directed at a specific color or color band, at one or more polarizations.
A common detector technology is photovoltaic (PV) which generally uses layers of different materials forming a PN or PIN junction at their interface. When exposed to a photon having energy equaling or higher than the band gap between the junction materials, the photon energy causes formation of electron-hole groups, which are separated and collected on both sides of the junction.
Another class of detectors employs polymer based photoabsorptive material as electron donors in combination with electron acceptors. In some cases the resulting excited electron is separated from the corresponding hole using different work function conductors. In other cases, a polymer electron acceptor forms a heterojunctions with the electron donor. Such detectors generally are less expensive than PV detectors, but have lower efficiency therefrom; however their efficiency may be significantly advanced by aspects of the present invention, so as to make them viable for the image array sensor applications. It is noted that the PN junction of PV detectors may be considered as an electron acceptor and electron donors materials respectively, even if the mechanisms by which the charges are separated may differ. Inserting an intrinsic layer between the electron donor and acceptor layer is a well known technique for increasing efficiency and directing energy level to its most advantageous location.
Other types of detectors utilize antennas, and more commonly rectennas, to achieve the energy conversion. The term rectenna relates to an antenna structure having a rectifier integrated with, or closely coupled to, the antenna, such that electromagnetic energy incident on the antenna is rectified and presented as primarily unidirectional (ideally DC) signal. By way of example, rectennas are described in U.S. Pat. No. 7,799,988 to Cutler, and in “Nanoscale Devices for Rectification of High Frequency Radiation from the Infrared through the Visible: A New Approach”, N. M. Miskovsky, P. H. Cutler, A. Mayer, B. L. Weiss, Brian Willis, T. E. Sullivan, and P. B. Lerner, Journal of Nanotechnology, Volume 2012, Article ID 512379, doi:10.1155/2012/512379, Hindawi Publishing Corporation©.
Other detector technologies may be utilized. Charge coupled devices, photoresistors, phototransistors, reverse biased LED's (light Emitting Diodes), photoconductors, CMOS, and the like, are all possible selections for one or more detectors, and the selection of a specific detector is a matter of technical choice. Certain embodiments of the present invention utilize PV, polymer, and rectenna based detectors. Optionally the detectors are disposed within waveguides.
Presently, the two most prevalent image sensor technologies are CCD (Charge Coupled Device), and CMOS (Complementary Metal Oxide Semiconductor), with some combinations thereof. Color sensors are generally obtained by either filtering colors into adjacent pixels, in a technique known as Bayer Filter, by using multi-layered pixels or by utilizing three separate detectors.
Bayer filters utilize pixel filters of different colors laid over adjacent pixels. In the Foveon 3X© system (Foveon© Inc., Santa Clara, Calif., USA), three different layers are stacked on top of each other, each layer being sensitive to one primary color. The stacked detector layers of the same bandgap or of differing bandgap have been shown to improve efficiency and image quality over filter based image sensors. Detecting higher frequency signals in a first, higher bandgap material and transmitting lower frequency waves with photon energy below the material bandgap allows their subsequent conversion in lower bandgap materials allow better light capture, reduces color artifacts, and simplifies processing.
Three separate detectors are generally used in high end applications. Incoming light is separated to the three primary colors, either by filters, prisms, dichroic mirrors and the like, and each primary color impinges on a single detector dedicated to that color. The use of three detectors, combined with the color separation system increases camera size, and is expensive using presently available methods.
Waveguides are a known structure for trapping and guiding electromagnetic energy along a predetermined path. An efficient waveguide may be formed by locating a layer of dielectric or semiconducting material with a higher dielectric constant between neighboring regions with lower dielectric constants. It may also be formed by bounding a layer of dielectric or semiconducting material between two conductive layers. Waveguides are easily formed using simple wafer scale processes; however such waveguides guide light parallel to the surface.
There is a long felt and heretofore unsolved need for better technology, providing inexpensive, sensitive high quality image array sensors, and preferably sensor technology which may be made broad band, and/or polarization sensitive.